1. Field of the Invention
The present invention relates to a method and apparatus for reading an original sheet by converting an original image to an electrical signal and, more particularly, to a method and apparatus for reading an original sheet by using a charge storage type photo sensor.
2. Description of the Prior Art
When an original sheet is fed intermittently and a photo sensor reads an original image asynchronously with original sheet feeding, the reading range or interval of the photo sensor will not be constant, thus degrading resolution.
In order to prevent this, as shown in FIG. 1, a conventional method has been proposed wherein a displacement of the original sheet has a periodic relationship with an output of the photo sensor in such a manner that the data charged by the photo sensor is not read while the original sheet is moved. Another conventional method has been proposed wherein the ratio of the displacement time of the original sheet to the charge time of the photo sensor is small.
Referring to FIG. 1, time T1 is the displacement time of the original sheet; and T2, the charge time of the photo sensor.
When high-speed reading of the original sheet is performed in accordance with such methods, the displacement time T1 of the original sheet or the charge time T2 of the photo sensor must be shortened. In order to shorten the displacement time T1 of the original sheet, an expensive stepping motor with a high torque is required. If this is used, the apparatus becomes heavy and generates much noise.
In order to shorten the charge time T2 of the photo sensor, the amount of light received by the photo sensor must be increased. For this purpose, a high-luminance light source is required, resulting in high power consumption.